The present invention generally relates to so-called metal-semiconductor field effect transistors (MESFET) having a gate electrode which makes a Schottky contact with a semiconductor layer underneath, and more particularly to a high power MESFET device for operation in a high frequency region.
A MESFET is a type of junction field effect transistor comprising a gate electrode, a source electrode and a drain electrode making contact with a semiconductor layer underneath, wherein the gate electrode makes a Schottky contact and the source and drain electrodes make ohmic contacts. Such a MESFET has a simple structure which is easily manufactured. Further, MESFET is suited for miniaturization as the gate length of such a device is easily decreased. A MESFET based on gallium arsenide (GaAs), which provides high electron mobility, is used for various high power devices for high frequency use as well as high speed integrated circuits in the field of microwave or satellite telecommunications.
In a typical MESFET structure, a semi-insulating semiconductor material such as intrinsic GaAs is used as a substrate and the GaAs substrate is doped to the n-type along its top surface to form an active layer in which a conductive channel is formed. On the active layer, there is provided a gate electrode so as to make Schottky contact therewith, and on opposite sides of the gate electrode, a source electrode and a drain electrode are provided so as to make ohmic contact with the active layer underneath. In such a MESFET structure, there is formed a depletion region in the active layer in correspondence to the gate electrode which limits the channel formed in the active layer. By applying a negative voltage to the gate electrode, the depletion region extends further into the active layer and the electrical current flowing through the channel from the drain electrode to the source electrode is reduced or shut down. By reducing the magnitude of the negative gate voltage or by applying a positive voltage to the gate electrode, on the other hand, the extension of the depletion region into the active layer is reduced and the channel in the active layer is extended. Responsive thereto, the electrical current flowing through the channel is increased.
In such a simple MESFET structure, there are formed additional depletion regions in the active layer, one in the surface part between the gate electrode and the source as well as one between the gate electrode and the drain electrode, as a result of the surface potential induced by the surface states. Such depletion regions also limit the dimensions of the channel in the active layer and as a result, the maximum electrical current flowing through the channel is limited. It should be noted that these additional depletion regions are not controlled as to the extension of each into the active layer, in contrast to the case of the depletion region formed underneath the gate electrode. Thus, the simple MESFET has a problem in that the maximum source-drain current is limited to a level lower than the level which the device is potentially capable of providing because of the additional depletion regions which result from surface states and tend to block, or pinch-off the conductive channel.
In order to avoid this problem, there is proposed a MESFET structure having a recessed gate in which a groove is provided in the active layer between the source electrode and the gate electrode by isotropic etching, and the gate electrode is provided on the bottom of the groove thus formed. By providing the gate electrode at a level substantially lower than the level of the source and drain electrodes on the surface of the substrate, the channel extending between the source electrode and drain electrode passes through a part of the active layer away from its surface and the deteriorative effect of the depletion region at the surface of the active layer is avoided. However, formation of the groove in the active layer by etching is undesirable as it is difficult to control the extent of etching exactly. Further, such a process complicates the manufacturing of the device. It should be noted that the etching of the groove determines the thickness of the active layer which is critical to the operation of the device, especially the drain current of the device which is determined by the channel.
Further, there is proposed another type of MESFET structure designed to avoid the deteriorative effect of the surface depletion region, as described in Japanese Laid-open Patent Application No. 58-50780, in which the surface of the active layer including the source, drain and gate electrodes is covered by an insulator layer and another electrode is provided on the insulator layer so as to cover the latter uniformly. Referring to FIG. 1 showing this prior art MESFET structure, the MESFET structure is formed on a substrate 1 of GaAs having a surface region 2 doped to the n-type. On the surface region 2, there are formed a pair of conductive layers 2a and 2b doped to the n.sup.+ -type and an active layer 2c doped to the n-type. In correspondence to the layers 2a, 2b and 2c, metal electrodes 3a, 3b and 3c forming the source, drain and gate electrodes are deposited as usual. Further, an insulator layer 4 is deposited on the surface region 2 so as to cover the electrodes 3a-3c and the active layer 2c. Furthermore, an electrode 5 is deposited so as to cover the insulator layer 4. According to this prior art MESFET structure, a depletion region 6 caused by the surface states of the active layer 2c is compensated or eliminated by applying a positive voltage to the additional electrode 5. Thus, one can eliminate the unnecessary limitation of the channel in the active layer 2c and the maximum source-drain current is increased.
In this MESFET structure, however, there arises a problem in that a substantial junction capacitance appears between the gate electrode 3c and the electrode 5 as well as between the source or drain electrode 3a, 3b and the electrode 5 as a result of the use of the insulator 4. Any of these capacitances causes a decrease in the gain of the device particularly in the high frequency range and the high frequency operational characteristic of the MESFET is deteriorated accordingly. Further, the provision of such an insulator layer 4 tends to invite unstable operation of the device such as oscillation particularly when silica (SiO.sub.2) or other oxide is used as the insulator layer 4. In such a case, oxygen in the SiO.sub.2 layer 4 tends to penetrate into the GaAs active layer 2c and form a deep impurity level. Further, the provision of the insulator layer 4 is not easy, as compared to the case of silicon transistors, since the layer 4 cannot be formed by simple oxidation of the surface of the substrate. When the insulator layer 4 is formed by deposition of silicon nitride (Si.sub.3 N.sub.4) which is another material commonly used for insulating layer, on the other hand, there is a problem of mechanical stress accumulated in the layer 4 particularly when the thickness of the layer 4 is increased. When the thickness of the layer 4 is decreased so as to decrease the mechanical stress, on the other hand, the capacitance between the various electrodes, as above-noted, is increased. Such an increase in capacitance, especially between the gate electrode 3c and the electrode 5, significantly deteriorates the operation of the MESFET in the high frequency region. In a commonly used Si.sub.3 N.sub.4 layer of about 1000 .ANG. thickness, there is accumulated a stress which is ten times as large as the stress caused in the corresponding SiO.sub.2 layer. Further, the structure of FIG. 1 is inconvenient for connection of wiring conductors to the gate, source or drain electrodes as these electrodes are located underneath the electrode 5.